1. Technical Field of the Invention
The present invention relates to a projection type display apparatus for enlarging an image formed by a liquid crystal light valve and projecting the enlarged image onto a screen.
2. Description of the Related Art
FIG. 25 illustrates an optical system of a prior art projection type display apparatus using a single liquid crystal light valve. Referring to FIG. 25, a light source 1 includes an elliptical mirror 12 and a white-light lamp 11. Disposed downstream of the light source 1 are a collimator lens 14, field lens 21, liquid crystal light valve 6, and projection lens 8 in this order, as illustrated in FIG. 25. The liquid crystal light valve 6 includes transparent substrates 62 and 64 and a liquid crystal layer 61 sandwiched between the transparent substrates 62 and 64.
The aforementioned prior art apparatus operates as follows: The light emitted from the white-light lamp 11 is reflected and converged by the elliptical mirror 12 into a light spot I which serves as a circular, plane light source, i.e., a secondary light source when seen from the optical system positioned downstream of the elliptical mirror 12. The light emitted from the spot I is then incident on the collimator lens 14 which in turn converts the incident light into a parallel illumination luminous flux 15. The field lens 21 converts the parallel illumination luminous flux 15 into a converged light beam which passes through the light valve 6. The light valve 6 is driven by a drive circuit to form an image therein in accordance with an input signal. The image from the light valve 6 is then incident upon the projection lens 8. The projection lens 8 enlarges the image and projects the enlarged image onto a screen.
FIG. 26 illustrates a prior art optical system of a projection type display apparatus using three liquid crystal valves (Y. Ooi et al., Proceedings of the 12th, International Display Research Conference, pp. 113-116 (1992)). Elements similar to those in FIG. 25 have been given similar reference numerals.
The prior art optical system of FIG. 26 operates as follows: The luminous flux emitted from the white-light lamp 11 is reflected and converged by the elliptical mirror 12 into the light spot I and is then reflected by a mirror 13 to a collimator lens 14. The collimator lens 14 converts the incident light into a parallel luminous flux 15 which is then separated into three primary colors, i.e., red, green, and blue by dichroic mirrors 2GR and 2G. The light of the primary colors is then incident on the liquid crystal light valves 6R, 6G, and 6B through field lenses 21R, 21G, and 21B, respectively. The blue light is reflected by a mirror 3 into the field lens 21B. The liquid crystal light valves 6R, 6G, and 6B are each driven by a drive circuit to produce red, green, and blue images, respectively.
The images of the primary colors are then combined into a full color single image by dichroic mirrors 70G, 70GB, and a mirror 4. The full color image is converted by a projection lens 8 into a projection light 9, which is projected as an enlarged full color image onto a screen 10 for viewing. The field lenses 21R, 21G, and 21B convert the parallel illumination luminous flux into converged light so that the light is efficiently incident upon the projection lens 8. The projection lenses 8 shown in FIGS. 25 and 26 may be provided with an aperture stop therein as required.
The aforementioned liquid crystal light valves 6R, 6G, and 6B are formed of liquid crystal material such as TN (Twisted Nematic) type, STN (Super Twisted Nematic) type, or polymer dispersion type liquid crystal. Polymer dispersion type liquid crystal includes PDLC (Polymer Dispersed Liquid Crystal), LCPC (Liquid Crystal Polymer Composite), NCAP (Nematic Curvilinear Aligned Phase), and PNLC (Polymer Network Liquid Crystal). Reference is made to "Conference Record of the 1991 International Display Research Conference, pp 215-218 (1991), by J. De Baets et al." and "SID '92 Digest, pp 575-578 (1992), by S. Niiyama et al." for details regarding polymer dispersion liquid crystals. In addition to the projection type display apparatus shown in FIGS. 25 and 26, a projection type display apparatus using a reflection type liquid crystal light valve is also known.
The problem with liquid crystal light valves using the aforementioned materials is that the characteristics of the material when the material is driven vary depending on their operating temperatures. The respective material has its own optimum operating temperature and its response becomes slower as temperature decreases below the optimum operating temperature. Slower response presents problems such as:
(1) the image resolution is deteriorated when displaying a moving picture, or PA1 (2) the preceding image remains superimposed on the following image for a while immediately after switching from one stationary picture to another.
Moreover, the material shows different electrooptic characteristic (relation between the transmittance of the material and the voltage applied to the material) at temperatures outside the optimum operating temperature range. The changes in electrooptic property results in deteriorated brightness and contrast in the displayed image.
It is known that the dispersive power of polymer dispersion type liquid crystals varies with voltage applied thereto and their electrooptic characteristics exhibit hysteresis. Recently, the materials used for polymer dispersion type liquid crystal 15 have been intensively developed in order to solve the hysteresis problem and materials have been developed which operate with practically no problems in displaying images when the materials are used at their optimum operating temperatures. However, the experiments conducted by the inventors of the present invention revealed that such improved materials still exhibited prominently increased hysteresis if they are operated at temperatures below their optimum temperature ranges. In experiment conducted, a liquid crystal projector was used which uses three liquid crystal light valves as shown in FIG. 26. The liquid crystal light valves are formed of LCPC having an optimum operating temperature range of from 30 to 50.degree. C. The projector was placed in an environment of room temperature below 10.degree. C. and was turned on to display a stationary image of a personal computer. When the stationary image is switched from one image to another, the preceding image remained superimposed on the following image for a short time due to hysteresis. In practice, this is a serious problem.